A new category of display technology is called the ILED (Inorganic Light Emitting Diode) display, an alternative to the better known LCD (Liquid Crystal Display) and the OLED (Organic Light Emitting Diode) displays. An ILED display does not have any of the negative qualities of LCD or OLED displays as it is inherently the same as a standard LED and has all the advantages thereof. It will have high quality black output, high quality white output, no dither, high quality uniformity, long lifespan, very low power consumption, predictable colour gamma, fast response rates and negligible flicker.
Projection display technology ranges from large area projection displays (screen displays) to miniature microdisplays such as headup imaging displays.
Projection displays generally consist of 4 components: a light source, an imaging engine, a driver and an optical path. The light source provides input light for the imaging engine. The imaging engine then manipulates the light to produce the image. The driver instructs the imaging engine how the required image should be produced—i.e. which pixels should be on or off. Finally, the optical path will expand, contract or otherwise control the image such that it is as specified by the system for the intended application.
The light required for projection can be generated as a continuous emission band (halogen, fluorescent bulbs, white LEDs) or in discrete bands (RGB LEDs, laser sources). The imaging engine options include Digital Light Processing (DLP), galvano-scanning mirrors, Liquid Crystal Displays (LCD) or Liquid Crystal on Silicon (LCOS). For example in LCD, the enabling engine is the liquid crystal plus a series of filters to control the light color. LCD displays however require a backlight which typically consists of an array of LED devices and a diffuser to provide uniform illumination. Similarly for large scale LED displays (sports stadia/shop mall screens), the light source and imaging engine are combined using large area ILED chips packaging in standard SMT formats. In the above examples the driver is the control electronics (in the form of an active backplane or a silicon controller chip) which decides the pixels to be activated based on incoming data (presumably from a graphical processing unit).
At the other end of the display scale size are micro-displays. In these devices either a very small image or a very small imaging system is targeted. A very small image may be used in retina projected displays (such as used in the Google Glass) while a small imaging system is required for other heads-up type displays. A range of image engines and light sources have been employed in such devices including those listed above. Of commercial interest today is OLED technology. OLED technology is comparable to large scale ILED displays where the light source and imaging engine are one in the same. However the manufacturing method while not capable to manufacture large scale displays equivalent to ILED is nevertheless capable of manfacturing high resolution displays with 250+ ppi equivalent to LCD technology. As with the displays above, the driver of an OLED display may be an active backplane or a silicon controller chip.
Both OLED and LCD type displays have significant drawbacks for achieving higher resolution pixel for micro-displays with, for example, sub 15 μm pixel pitch. For example, OLED's have limited resolution because of the shadow mask manufacturing process which limits the resolution to <300 ppi. Methods to overcome this include pentile emitter design configurations for R, B and G chips to enhance the resolution of the OLED display to 300+ ppi. This design overcomes the resolution issues associated with shadow mask manufacture with OLEDs and eliminates uniformities with TFT's to produce an overall smooth defect free image.
Other methods exist to achieve higher resolution displays by monolithic array manunfacturing methods. Monolithic is used to refer to a component that is indivisible from another and is formed from a larger block. The term “monolithic array” refers to a light emitting device that has several addressable emitting areas which are fabricated on the same material and are physically connected. A monolithic array is particular to Inorganic LED devices and is not formed from OLED materials. This is due to the difference in manufacturing of the two LED types. Inorganic LEDs are fabricated using a “top-down” method. In this method, the starting point is a singular piece of semiconductor material from which the LEDs are produced. If physical contact between a number of light emitting areas remains at the end of the fabrication processes (i.e. the semiconductor material has not be separated), then it can be said that a monolithic array has been produced. The monolithic ILED array chip is a singular piece of semiconductor material in which multiple emitting areas are formed. This is distinct from the assembly of light emitting devices on a physical interconnect that allows them to be transposed in unison.
In contrast, organic LEDs are fabricated by a “bottom-up” approach. This occurs by the deposition of organic materials in repeated layers on a target substrate. Since an OLED device does not start from singular piece of semiconductor material, the finished device cannot be considered monolithic. It is noted that in certain publications, monolithic OLEDs may be referred to. However, this is related to the integration of the OLED device directly on a driver circuit, commonly a CMOS. In this case, the backplane or CMOS chip, which has been formed from a “top-down” approach, is monolithic and the OLED material has been is integrated with it to form a monolithic component.
The fabrication of monolithic ILED devices is necessiated because of the need to avoid chip manufacture and micropostioning at the scale of 10 μm or less. Examples can include the fabrication of chips containing as an example 160×120 individually addressable pixels on a mono-lithic chip >1 mm×1 mm. Displays based on large monolithic ILED chips have inherent challenges. Firstly, since the ILED device begins from a singular piece of semiconductor material, the monolithic ILED array chip can only produce a single wavelength of light. In addition, the fabrication of monolithic ILED array devices produces issues assoicated with yields. When ILED devices are fabricated there will be a number of devices that are failed. When singular devices are fabricated then only the failed devices are discarded. However, with monolithic ILED array chips, a failed device will result in the discarding of the whole monolithic chip. In the example above of a 160×120 monolithic array, the failure of a single device results in the discarding of 19,200 emitters—the majority of which may be functional. This results in lower yields based on material used and fabrication quality.
It is important to note the distinct challenges associated with ILED and with OLED devices. For the fabrication of ILED array chips, the fabrication of features sizes of 1 μm (or smaller using nano-imprint lithographic techniques) is achievable. However, the ability to pick-and-place or micro-assemble ILED devices of sizes smaller that 10×10 μm2 (100 μm2) remains a very significant challenge. For OLED devices, the ability to form devices of sizes less than 10 μm is a challenge due to shadow masking and other effects associated with the deposition processes used. However, since OLED devices can be fabricated directly onto the target control devices (such as a CMOS or TFT backplane) there is no requirement to pick-and-place the devices after fabrication. OLEDs can be white and use filters.
Monolithic methods for high resolution active matrix monochromatic displays on silicon have been presented U.S. Pat. No. 8,557,616 B2 for LED technology. In this document, a singular piece of ILED material is integrated with the driver circuitry and results in a device which produces a single colour.